Voltage regulators are commonly used to convert unregulated (e.g., potentially varying and noisy) input voltages to regulated (e.g., relatively stable and noise-free) output voltages. A Low Dropout (LDO) voltage regulator is a particular type of linear voltage regulator, which is used when it is desirable to minimize the voltage drop between the regulator's input and output terminals (e.g., to as little as a few hundred millivolts or less). For example, a typical LDO voltage regulator includes a pass transistor having first and second current carrying terminals coupled to an unregulated input voltage terminal and a regulated output voltage terminal, respectively. The difference between the voltage across the regulator's output terminals (or the “regulated” voltage) and a reference voltage (produced based on the input voltage) is used to control the pass transistor (i.e., via the pass transistor's control terminal) in order to maintain a desired regulated voltage. Higher gain in this feedback loop (referred to as “loop gain”) enhances output voltage regulation accuracy, but makes maintaining system stability more difficult.
A load coupled across an LDO voltage regulator's output terminals may be characterized, for example, as a parallel combination of a variable load resistance and a variable load capacitance, where the load capacitance has a variable effective series resistance (ESR) associated with it. The variations in the load's resistance, capacitance, and ESR may result, for example, from any combination of temperature fluctuations, component variations, load configuration changes, and so on.
An LDO voltage regulator is capable of rapidly adjusting its output current (via modulation of the signal provided to the pass transistor) in the face of significant load variations to maintain a desired regulated voltage. However, the high open loop output impedance of a typical LDO voltage regulator makes the regulator's frequency stability particularly susceptible to such load variations, and absent appropriate compensation, the load variations may adversely affect the regulator's frequency stability. In modern circuits, a typical LDO voltage regulator may have many poles and zeros, and the feedback loops in such LDO voltage regulators may be very difficult to compensate.